The present invention relates to a bending machine and method for the bending of metal sheets and, in particular, for the bending of printing plates for use in printing machines.
Bending machines for the bending of metal sheets, but also special machines for the bending of printing plates, which are employed particularly in offset printing, are sufficiently known from the prior art. Thus, for example, it may be gathered from the German patent publication no. DE 33 40 144 C2 that particularly high requirements as to accuracy apply to the bending of the sheet-metal edges of printing plates. This is because the bending edges of the printing plates must be capable of being introduced into the tensioning channel on the outer surface area of the plate cylinder of the printing machine and of being positioned there as accurately as possible.
FIG. 1 shows diagrammatically a partial cross-section through the set-up of a conventional bending machine. Reference symbol 1 is a bending table which is equipped with a bending cheek. On this lies the printing plate 2 to be bent, which is pressed onto the bending table by a holding-down plate 3 in the region of the pressure pad 4. When the printing plate 2 is clamped firmly, the end of the printing plate 2 is bent to form an edge by means of the bending bar 6. This bending operation must take place as exactly as possible for the entire printing plate, since, if the bending edges are not executed exactly on both longitudinal sides of the printing plate, that is to say the edges on the leading and the trailing side of the printing plate, a reduced printing quality is obtained in the case of high printing speeds.
Accordingly, for the present invention, the object arises of providing a bending machine by means of which it is possible to produce bending edges, particularly on a printing plate, with high accuracy.
According to a first aspect of the invention, a bending machine for the bending of a printing-machine printing plate is provided, the bending machine having a bending table, on which the metal sheet or printing-machine printing plate to be processed is deposited for the bending operation. By means of a pressure pad which is fastened to a holding-down plate, the printing plate is fixed to the bending table. Fixing takes place in the immediate vicinity of the bending edge to be produced, before a bending bar bends the metal sheet via a bending cheek having the corresponding angle provided for the metal sheet. The holding-down plate is brought directly up to the metal sheet via a corresponding mechanism, so that a defined clearance is obtained between the pressure pad and the metal sheet to be bent. The pressure pad fastened to the holding-down plate can execute a relative movement with respect to the holding-down plate. By virtue of this relative movement, it is possible to carry out a finely adjusted fixing of the metal sheet on the bending table. This finely adjusted fixing takes place, via the pressure-pad surface, more uniformly than was possible in the prior art, so that the metal sheet is held with a more uniformly distributed force along the bending edge to be produced. A more accurate bending of the metal sheet consequently becomes possible.
According to an advantageous refinement of the invention, a linear drive is provided for executing the relative movement of the pressure pad with respect to the holding-down plate. The linear drive can advantageously introduce as uniformly as possible, over the entire length of the pressure pad, the clamping force which is necessary for fixing the printing plate. This is because, for the idea of the invention, it is essential that as identical a clamping force as possible acts over the entire length of the edge to be bent. Different clamping forces along the bending edge to be produced would also lead to differently pronounced edge portions.
According to an advantageous refinement, the linear drive used is a stretchable hose which preferably consists of silicone and which is filled with a fluid in order to introduce the clamping force. The fluid used may be, for example, compressed air. The hose is arranged between the holding-down plate and the pressure pad and is supported on the holding-down plate, in order to transmit the clamping force, which occurs due to the expansion of the hose, to the metal sheet to be bent. In order to bring about as uniform a force transmission as possible over the entire length of the pressure pad, it is expedient that the hose runs between the holding-down plate and the pressure pad in the longitudinal direction of the pressure pad, that is to say runs in the direction of and along the edge to be bent. However, instead of this arrangement, other geometries may also be used for the run of the hose, insofar as merely the principle of uniform force transmission is implemented. The material used for the hose to be employed is preferably silicone, but all materials allowing sufficient radial expansion may be envisaged.
According to a further refinement according to the invention, a pressure pad is constructed in two parts and consists of a spring element and of a plastic pad. The two-part construction makes it possible for the pressure pad to be fastened to the holding-down plate, without the pressure pad itself having to be capable of being subjected to bending stress. However, it is also conceivable to select the material of the pressure pad such that the latter can be fastened to the holding-down plate and can be pressed away from the holding-down plate by means of the hose.
In the case of a two-part construction, there is provision for the spring element to be a sheet of spring steel which on one longitudinal side, which corresponds to the length of the edge to be bent, is fastened to the holding-down plate, for example by means of screws, and on the other longitudinal side has the plastic pad which is then pressed via the expanding hose against the metal sheet or printing plate to be bent.
Instead of the two-part construction with a spring element, various designs may be envisaged which make it possible for an element acting as a pressure pad to be fastened movably to the holding-down plate, so that a relative movement between the holding-down plate and pressure pad becomes possible.
According to a further refinement of the invention, the hose is provided in a groove of the holding-down plate, so that a defined linear movement in the direction of the metal sheet to be bent becomes possible. The groove is preferably designed with a depth such that, in the state of rest, the hose is accommodated completely in the holding-down plate and the pressure pad comes to bear positively against the holding-down plate. However, this set-up is not mandatory, and it is also conceivable to have a design in which the hose is accommodated in the pressure pad, so that further machining of the holding-down plate no longer becomes necessary. In order to set the defined clearance between the holding-down plate and the metal sheet to be bent, it is advantageous, furthermore, to provide a spacer which may be implemented, for example, by means of a stop screw which makes it possible to set a variable clearance between the holding-down plate and the metal sheet to be bent or the bending table.